The present invention relates to a double clutch transmission for motor vehicles, with six or seven gears.
A six-gear double clutch transmission for motor vehicles is known from Italian patent application No. TO2001A000286 in the name of the applicant. This known arrangement enables all the gear changes to be made in the so-called power-shift mode, that is to say, with controlled transfer of power and torque between the two friction clutches.
The above-mentioned double clutch transmission for motor vehicles suffers from the disadvantage of having a different structure from that of ordinary single clutch transmissions, whether manual or robotized. Therefore, this double clutch transmission cannot be easily produced from the associated single clutch transmission but requires dedicated production systems, with a consequent increase in manufacturing costs.
A further example of a six-gear double clutch transmission for motor vehicles is known from FR 2 802 600. According to this known arrangement, the transmission comprises:                a first input shaft carrying a driving gearwheel associated to both the first gear and the reverse gear, a driving gearwheel associated to the third gear and a driving gearwheel associated to the fifth gear,        a second input shaft made in the form of a hollow shaft coaxial with the first and carrying a driving gearwheel associated to the second gear and a driving gearwheel associated to both the fourth gear and the sixth gear,        a first output shaft carrying driven gearwheels of first, third, fifth and sixth gear, as well as a first engagement sleeve for selectively engaging the first or the third gear and a second engagement sleeve for selectively engaging the fifth or the sixth gear, and        a second output shaft carrying driven gearwheels of reverse, second and fourth gear, as well as a third engagement sleeve for engaging the reverse gear and a fourth engagement sleeve for selectively engaging the second or the fourth gear.        
Also this arrangement does not allow shifting between the fifth and the sixth gear in power-shift mode. However, this limitation does not adversely affect the driving comfort, since the jerk occurring in the vehicle upon shifting between the two highest gears is contained and therefore scarcely noticeable by the driver.
On the other hand, this known architecture is not well suitable for a seven-gear double clutch transmission. In fact, switching from a six-gear to a seven-gear transmission while retaining the advantage of enabling all the gear changes to be made in the power-shift mode, except for that between the two highest gears, would involve an increase in the axial size of the transmission, since the driven gearwheel of seventh gear could not be placed on the second output shaft in the free space between the driven gearwheels of reverse and second gear but should be placed on the first output shaft next to the driven gearwheel of sixth gear.